Twin plasma torch apparatus

ABSTRACT

A twin plasma torch assembly includes two plasma torch assemblies supported in a housing. Each of the two plasma torch assemblies has an electrodes. Plasma gas is introduced into a processing zone around the two electrodes. A shroud gas is introduced to surround the plasma. A feed tube is provided to supply feed material to the processor.

The invention relates to a twin plasma torch apparatus.

In a twin plasma torch apparatus, the two torches are oppositely chargedi.e. one has an anode electrode and the other a cathode electrode. Insuch apparatus, the arcs generated by each electrode are coupledtogether in a coupling zone remote from the two torches. Plasma gasesare passed through each torch and are ionised to form a plasma whichconcentrates in the coupling zone, away from torch interference.Material to be heated/melted may be directed into this coupling zonewherein the thermal energy in the plasma is transferred to the material.Twin plasma processing can occur in open or confined processing zones.

Twin plasma apparatus are often used in furnace applications and havebeen the subject of previous patent applications, for example EP0398699and U.S. Pat. No. 5,256,855.

The twin arc process is energy efficient because as the resistance ofthe coupling between the two arcs increases remote from the two torches,the energy is increased but torch losses remain constant. The process isalso advantageous in that relatively high temperatures are readilyreached and maintained. This is attributable to both the fact that theenergy from the two torches is combined and also because of the abovementioned efficiency.

However, such processes have disadvantages. If the plasma torches are inclose proximity to one another and/or are enclosed within a small space,there is a tendency for the arcs to destabilise, particularly at highervoltages. This side-arcing occurs when the arcs preferentially attachthemselves to lower resistance paths.

The problem of side-arcing in current twin torch apparatus has lead tothe development of open processing units in which the plasma torches aresubstantially spaced apart, with low resistance paths removed fromvicinity, as described in U.S. Pat. No. 5,104,432. In such units, theprocess gas is free to expand in all directions in these applications.However, such arrangements are not suitable for all processingapplications, particularly when expansion of process gases needs to becontrolled e.g. production of ultra fine powders.

In current systems with confined processing zones, the torch nozzlesproject into the chamber so that the chamber walls, which have a lowresistance, are removed from the vicinity of the plasma arc. Thisawkward construction inhibits side-arcing and encourages coupling of thearcs. However, the protruding nozzles provide surfaces on which meltedmaterial may precipitate. This not only results in wastage of materialbut shortens the life of the torches.

The present invention provides a twin plasma torch assembly comprising:

(a) at least two twin plasma torch assemblies of opposite polaritysupported in a housing, said assemblies being spaced apart from oneanother and each comprising

(i) a first electrode,

(ii) a second electrode which is or is adapted to be spaced apart fromthe first electrode by a distance sufficient to achieve a plasma arctherebetween in a processing zone;

(b) means for introducing a plasma gas into the processing zone betweenthe first and second electrodes;

(c) means for introducing shroud gas to surround the plasma gas;

(d) means for supplying feed material into the processing zone; and

(e) means for generating a plasma arc in the processing zone.

The shroud gas confines the plasma gas, inhibits side-arcing, andincreases plasma density. The invention therefore provides an assemblyin which the torches are inhibited from side-arcing, and thusfacilitates the miniaturisation of torch design where distance to lowresistance paths are small. The use of shroud gas can also eliminate theneed for torch nozzles to extend beyond the housing.

The shroud gas may be provided at various locations along theelectrodes, particularly in cylindrical torches where arcs are generatedalong the length of the electrodes. However, preferably, each torch hasa distal end for the discharge of plasma gas and the means for supplyingshroud gas provides shroud gas downstream of the distal end of eachelectrode. Therefore, reactive gases such as oxygen may be added to theplasma without degrading the electrode. The practical applicability ofplasma torches is increased by the facility to add reactive gasesdownstream of the electrode.

In a preferred embodiment, each plasma torch comprises a housing whichsurrounds the electrode to define a shroud gas supply duct between thehousing and the electrodes, wherein the end of the housing is taperedinwards towards the distal end of the torch to direct flow of the shroudgas around the plasma gas.

The twin plasma torch assembly of the present invention may be used inan arc reactor having a chamber to carry out a plasma evaporationprocess to produce ultra-fine (i.e. sub-micron or nano-sized) powders,for example aluminium powders. The reactor may also be used in aspherodisation process.

The chamber will typically have an elongate or tubular form with aplurality of orifices in a wall portion thereof, a twin plasma torchassembly being mounted over each orifice. The orifices, and thus thetwin plasma torch assemblies, may be provided along and/or around saidtubular portion. The orifices are preferably provided at substantiallyregular intervals.

The distal ends of the first and/or second electrodes, for the dischargeof plasma gas will typically be formed from a metallic material, but mayalso be formed from graphite.

The plasma arc reactor preferably further comprises cooling means forcooling and condensing material which has been vaporised in theprocessing zone. The cooling means comprises a source of a cooling gasor a cooling ring.

The plasma arc reactor will typically further comprise a collection zonefor collecting processed feed material. The process feed material willtypically be in the form of a powder, liquid or gas.

The collection zone may be provided downstream of the cooling zone forcollecting a powder of the condensed vaporised material. The collectionzone may comprise a filter cloth which separates the powder particulatefrom the gas stream. The filter cloth is preferably mounted on anearthed cage to prevent electrostatic charge build up. The powder maythen be collected from the filter cloth, preferably in a controlledatmosphere zone. The resulting powder product is preferably then sealed,in inert gas, in a container at a pressure above atmospheric pressure.

The plasma arc reactor may further comprise means to transport processedfeed material to the collection zone. Such means may be provided by aflow of fluid, such as, for example, an inert gas, through the chamber,wherein, in use, processed feed material is entrained in the fluid flowand is thereby transported to the collection zone.

The means for generating a plasma arc in the space between the first andsecond electrodes will generally comprise a DC or AC power source.

The apparatus according to the present invention may operate withoutusing any water-cooled elements inside the plasma reactor and allowsreplenishment of feed material without stopping the reactor.

The means for supplying feed material into the processing zone may beachieved by providing a material feed tube which is integrated with thechamber and/or the twin torch assembly. The material may be particulatematter such as a metal or may be a gas such as air, oxygen or hydrogenor steam to increase the power at which the torch assembly operates.

Advantageously, the distal ends of first and second electrodes, for thedischarge of plasma gas, do not project into the chamber.

The small size of the compact twin torch arrangement according to thepresent invention allows many units to be installed onto a producttransfer tube. This enables easy scale-up to typically over 10 times togive a full production unit without scale up uncertainty.

The present invention also provides a process for producing a powderfrom a feed material, which process comprises:

(A) providing a plasma arc reactor as herein defined;

(B) introducing a plasma gas into the processing zones between the firstand second electrodes;

(C) generating a plasma arc in the processing zones between the firstand second electrodes;

(D) supplying feed material into the plasma arcs, whereby the feedmaterial is vaporised;

(E) cooling the vaporised material to condense a powder; and

(F) collecting the powder.

The feed material will generally comprise or consist of a metal, forexample aluminium or an alloy thereof. However, liquid and/or gaseousfeed materials can also be used. In the case of a solid feed, thematerial may be provided in any suitable form which allows it to be fedinto the space between the electrodes, i.e, into the processing zone.For example, the material may be in the form of a wire, fibres and/or aparticulate.

The plasma gas will generally comprise or consist of an inert gas, forexample helium and/or argon.

The plasma gas is advantageously injected into the space between thefirst and second electrodes, i.e. the processing zone.

At least some cooling of the vaporised material may be achieved using aninert gas stream, for example argon and/or helium. Alternatively, or incombination with the use of an inert gas, a reactive gas stream may beused. The use of a reactive gas enables oxide and nitride powders to beproduced. For example, using air to cool the vaporised material canresult in the production of oxide powders, such as aluminium oxidepowders. Similarly, using a reactive gas comprising, for example,ammonia can result in the production of nitride powders, such asaluminium nitride powders. The cooling gas may be recycled via awater-cooled conditioning chamber.

The surface of the powder may be oxidised using a passivating gasstream. This is particularly advantageous when the material is areactive metal, such as aluminium or is aluminium-based. The passivatinggas may comprise an oxygen-containing gas.

It will be appreciated that the processing conditions, such as materialand gas feed rates, temperature and pressure, will need to be tailoredto the particular material to be processed and the desired size of theparticles in the final powder.

It is generally preferable to pre-heat the reactor before vaporising thesolid feed material. The reactor may be preheated to a temperature of atleast about 2000° C. and typically approximately 2200° C. Pre-heatingmay be achieved using a plasma arc.

The rate at which the solid feed material is fed into the channel in thefirst electrode will affect the product yield and powder size.

For an aluminium feed material, the process according to the presentinvention may be used to produce a powdered material having acomposition based on a mixture of aluminium metal and aluminium oxide.This is thought to arise with the oxygen addition made to the materialduring processing under low temperature oxidation conditions.

Specific embodiments of the present invention will now be described indetail with reference to the following figures (drawn approximately toscale) in which:

FIG. 1 is a cross section of a cathode torch assembly;

FIG. 2 is a cross section of an anode torch assembly;

FIG. 3 shows a portable twin torch assembly comprising the anode andcathode torch assemblies of FIGS. 1 and 2, mounted onto a confinedprocessing chamber;

FIG. 4 shows the portable twin torch assembly of FIG. 3 mounted into ahousing;

FIG. 5 is a schematic of the assembly of FIG. 3 when used to produceultra fine powders;

FIG. 6A is a schematic of the assembly of FIG. 4 configured to operatein transferred arc to arc coupling mode, with a anode target;

FIG. 6B is a schematic of the assembly of FIG. 4 configured to operatein transferred arc mode, with a anode target;

FIG. 7A is a schematic of the assembly of FIG. 4 configured to operatein transferred arc to arc coupling mode, with a cathode target;

FIG. 7B is a schematic of the assembly of FIG. 4 configured to operatein transferred arc mode, with a cathode target.

FIGS. 1 and 2 are cross sections of assembled cathode 10 and anode 20torch assemblies respectively. These are of modular construction eachcomprising an electrode module 1 or 2, a nozzle module 3, a shroudmodule 4, and a electrode guide module 5.

Basically, the electrode module 1, 2 is in the interior of the torch 10,20. The electrode guide module 5 and the nozzle module 3 are axiallyspaced apart surrounded the electrode module 1, 2 at locations along itslength. At least the distal end (i.e. the end from which plasma isdischarged from the torch) of the electrode module 1, 2 is surrounded bythe nozzle module 3. The proximal end of the electrode module 1 or 2 ishoused in the electrode guide module 5. The nozzle module 3 is housed inthe shroud module 4.

Sealing between the various modules and also the module elements isprovided by “O” rings. For example, “O” rings provide seals between thenozzle module 3 and both the shroud module 4 and electrode guide module5. Throughout the figures of the specification, “O” rings are shown assmall filled circles within a chamber.

Each torch 10, 20 has ports 51 and 44 for entry of process gas andshroud gas respectively. Entry of process gas is towards the proximalend of the torch 10, 20. Process gas enters a passage 53 between theelectrode 1 or 2 and the nozzle 3 and travels towards the distal end ofthe torch 10, 20. In this particular embodiment, shroud gas is providedat the distal end of the torch 10, 20. This keeps shroud gas away fromthe electrode and is particularly advantageous when using a shroud gaswhich may degrade the electrode modules 1, 2, e.g. oxygen. However, inother embodiments, the shroud gas could enter towards the proximal endof the torch 10, 20.

The shroud module 4 is fitted at the distal end of the torch 10, 20. Theshroud module 4 comprises a nozzle guide 41, a shroud gas guide 42, anelectrical insulator 43, a chamber wall 111, and also a seat 46. An “O”ring is provided to seal the chamber wall 111 and the nozzle guide 41.Optionally, coolant fluid may also be transported within the chamberwall 111.

The electrical insulator 43 is located on the chamber wall 111 such thatthere is no low resistance path at the distal end of the torch tofacilitate arc destabilisation. The electrical insulator 43 is typicallymade of boron nitride or silicon nitride.

The shroud gas guide 42 is located on the electrical insulator 43 andprovides support for the distal end of the nozzle module 3 and alsoallows flow of shroud gas out of the distal end of the torch. It istypically made from PTFE.

The nozzle guide 41 is made of an electrical insulator, such as PTFE,and is used to locate the nozzle module 3 in the shroud module 4. Thenozzle guide 41 also contains a passage 44 through which shroud gas isfed to an chamber 47. Shroud gas exits from the chamber 47 throughpassages 45 located in the shroud gas guide 42. These passages 45 arealong the contact edge with the electrical insulator 43.

Although shroud gas is shown to be delivered to the torch 10, 20 using aspecific arrangement for the shroud gas module 4 (FIG. 8), delivery maybe by other means. For example, shroud gas may be delivered near theproximal end of the torch, through a passage surrounding the process gaspassage 51. The shroud gas may also be delivered to an annular ringlocated at and offset from the distal end of the torch.

The electrode guide module 5 conveniently provides a passage or port 51for the entry of process gas. The internal proximal end of the nozzlemodule 3 is advantageously chamfered to direct flow of process gas fromthe passage 51 into the nozzle module 3 and around the electrode.

The electrode guide module 5 needs to be correctly circumferentiallyaligned such that the electrode guide cooling circuit and the torchcooling circuit (discussed below) align.

The nozzle module 3 and electrode modules 1 and 2 have cooling channelsfor the circulation of cooling fluid. The cooling circuits are combinedinto a single circuit in which cooling fluid enters the torch through ansingle torch entry port 8 and exits torch out of a single torch exitport 9. The cooling fluid enters through the entry port 8 travelsthrough the electrode module 1, 2 to the nozzle module 3, and then exitsout of the torch through a nozzle exit port 9. The fluid which leavesthe nozzle exit port 9 is transported to a heat exchanger to providecooled fluid which is recirculated to the entry port 8.

Looking at the flow of cooling fluid through the modules in detail,fluid entering from the torch entry port 8 is directed to an electrodeentry port 81. Cooling fluid enters the electrode near its proximal endand travels along a central passage to the distal end wherein it isredirected back to flow along a surrounding outer passage (or number ofpassages) and out of an electrode exit port 91. This fluid enters thenozzle at entry port 82 and flows along interior passages to the distalend of the nozzle. It is then directed back along surrounding passagesto the exit from the nozzle port 92. The fluid is directed to the torchexit port 9.

Any fluid which acts as an effective coolant may be used in the coolingcircuit. When water is used, the water should preferably be de-ionisedwater to provide a high resistance path to current flow.

The torches 10 and 20 may be used for twin plasma torch assemblies, inboth open and confined processing zone chambers. The construction ofconfined processing zone twin plasma torch assembly 100 is shown in FIG.9.

The assembly 100 is configured to provide torches 10, 20 which areeasily installed to the correct position for operation. For example, theoffset between the distal ends of the electrodes 1, 2 and the anglebetween them are determined by the dimensions of the assemblycomponents.

The torch and assembly modules are constructed to close tolerance toprovide good fitting between the modules. This would limit radialmovement of one module within another module. To allow ease of assemblyand re-assembly, corresponding modules would slide into one another andbe locked in by for example, locking pins. The use of locking pins inthe modules would also ensure that each module was correctly orientedwithin the torch assemblies ie. provide circumferential registration.

The confined processing zone twin torch assembly 100 comprises a cathodeand anode torch assemblies 10 and 20, and a feed tube 112. Typically,the two torches are at right angles to one another. The components arearranged to provide a confined processing zone 110 in which coupling ofthe arcs will occur. The feed tube 112 is used to supply powder, liquid,or gas feed material into the processing zone 110. The walls 111 of theshroud modules 4 conveniently define the chamber which contains theconfined processing zone 110.

The walls 111 provide a divergent processing zone 110 in which the lowresistance wall surfaces are maintained away from the arcs, inhibitingside-arcing. In addition, the divergent nature of the design allows gasexpansion after plasma coupling, without a constrictive pressurebuild-up.

The walls 111 define a conical chamber which may comprise curved or flatwalls. The perimeter of the walls 111 may be joined to chamber walls 113to enable the assembly 100 to be mounted (FIG. 4). In such anarrangement, there should obviously be an orifice 114 such that theprocessing zone 110 is not totally enclosed. Typically, a circularorifice 114 can have a diameter of 15 cm.

The confined processing zone 110 may be made as a separate modulecomprising the feed tube 112, and the chamber walls 111 and 113.

The assembly 100 may be mounted into a cylinder which comprises(optional) inner cooling walls 115, surrounded by an outer refractorylining 116 (FIG. 4). The lining 116 would preferably be a heat resistantmaterial. The walls 111 may themselves also have integrated coolingchannels.

Turning now to the operation of the torches 10, 20, a shroud gas isprovided to encircle the arcs generated from the electrodes. The shroudgas may be helium, nitrogen or air. Any gas which provides a highresistance path to prevent the arc from travelling through the shroud issuitable. Preferably, the gas should be relatively cold. The highresistance path of the shroud gas concentrates the arc into a relativelynarrow bandwidth. The tapered distal end of the nozzle module assists inproviding a gas shroud which is directed to encircle the arc.

The shroud gas also acts to confine the plasma and inhibits melted feedmaterial from being recirculated back towards the feed tube 112 or thechamber walls 111. Thus, the efficiency of processing is increased.

As the distal end of the nozzle no longer protrudes into the confinedprocessing zone, precipitation of melted feed material on the nozzle isinhibited. Thus, the operational life of the nozzle is prolonged, andthe efficiency of the material processing increased.

Any regions of the assembly which are particularly close to the arcs aremade or coated with an electrical insulator, for example the shroud gasguide 42 and the electrical insulator 43.

The invention may be applied to numerous practical applications, forexample to manufacture nano-powders, spherodisation of powders or thetreatment of organic waste. Some further examples are given below.

1. Gas Heater/Steam Generator

Due to the modular nature, the invention allows replacement of existinggas fossil fuel burners with an electrical gas heater. Introducing waterbetween the two torches will enable steam to be generated which may beused to heat existing kilns and incinerators. Gasses may be introducedbetween the arcs to give an efficient gas heater.

2. Pyrolysis/Gas Heating and Reforming

Introduction of liquid and/or gas, and/or solids into the coupling zonewill enable thermal treatment.

3. Reactive Material Processing

Materials which dissociate into chemically reactive materials may beprocessed in the unit as there need not be any reactor wall contact athigh temperatures.

In such cases, the walls 111 of the water cooled processing zone chamberwould have a grated surface to allow transpiration to occur. Thiscreates a protective barrier to stop reactive gas impingement.

4. Ultra-fine Powder Production

The assembly may be utilised to produce ultra fine powders (generally ofunit dimension of less than 200 nanometres) is illustrated in FIG. 5.The small size of the unit enables easy attachment of a quench ring 130in close proximity to the gaseous high temperature plasma coupling zone.Fine powder is produced in the zone 132, within the expansion zone 131.Higher gas quench velocities produce smaller the terminal unit dimensionof the particles.

A plurality of twin torch assemblies as herein described may be mountedon a processing chamber.

It is expected that the nano-powders produced by this method wouldproduce finer powders as it would be possible to install the quenchapparatus 130 in close proximity to the arc to arc coupling zone. Thiswould minimise the time available for the powder/liquid feed materialparticles to grow.

It will be appreciated that composite materials may be fed to makenano-alloy materials.

Introduction of fine powders, gasses or liquids between the arc willvaporize them and the vapor may then be quenched/and or reacted to givea powder of nano-sized powders.

5. Coupled or Transferred Arc Mode

The modular assembly may also be configured as to operate in transferredarc modes with anode (FIG. 6) and cathode (FIG. 7) targets. The torchesdescribed above are suitable for operation in transferred arc to arccoupling mode (FIGS. 6A and 7A) and transferred arc mode (FIGS. 6B and7B).

6. Spherodisation

Typical plasma gas temperatures at the arc to arc coupling zone havebeen measured to be up to 10,000 K for an Argon plasma. Introduction ofangular particles results in spherodisation.

7. Thermal Modification/Etching/Surface Modification

The Coupling zone between the arcs may be used to thermally modify afeed gas, for example methane, ethane or UF6.

The plasma plume may also be used to achieve surface modification by,for example, ion impingement, melting, or to chemically alter thesurface such as in nitriding.

8. ICP Analyses

The assembly according to the present invention may also be used in ICPanalyses and as a high energy UV light source.

Various modifications can be made to the above embodiments. For example,cooling water systems of the two torches may be combined, or one or bothof the torches of the twin apparatus could have a gas shroud. Inaddition, the gas shroud may be applied to torches which do not have themodular construction mentioned above.

The apex cone angle in the torch assembly may be different for differentapplications. In some cases it may be desirable to fit to a cylinderwithout a cone.

A plurality of twin torch assemblies as herein described may be mountedon chamber.

What is claimed is:
 1. A twin plasma torch assembly comprising: (a) atleast two plasma torch assemblies of opposite polarity supported in ahousing, said assemblies being spaced apart from one another andcomprising (i) a first electrode, (ii) a second electrode which is or isadapted to be spaced apart from the first electrode by a distancesufficient to achieve a plasma arc therebetween in a processing zone;(b) a passage for introducing a plasma gas into the processing zonearound each electrode; (c) a further passage for introducing shroud gasto surround the plasma gas; (d) a feed tube for supplying feed materialinto the processing zone; and (e) a power source for generating a plasmaarc in the processing zone; characterised in that distal ends of firstand second electrodes do not project beyond the housing.
 2. A twinplasma torch assembly as claimed in claim 1, wherein each torch has adistal end for the discharge of plasma gas, wherein the further passagefor supplying shroud gas provides shroud gas downstream of the distalend of each electrode.
 3. A twin plasma torch assembly as claimed inclaim 2, wherein each torch comprises a housing which surrounds theelectrodes to define the shroud gas supply duct between the housing andthe electrodes, and wherein the end of the housing is tapered inwardstowards the distal end of the torch to direct flow of the shroud gasaround the plasma gas.
 4. An assembly as claimed in claim 1, furthercomprising a collection zone for collecting processed feed material inthe form of a powder.
 5. An assembly as claimed in claim 4, furthercomprising means to transport processed feed material to the collectionzone.
 6. An assembly as claimed in claim 5, wherein the means totransport processed feed material to the collection zone comprises meansto provide a flow of fluid through the chamber, wherein, in use,processed feed material is entrained in the fluid flow and is therebytransported to the collection zone.
 7. An assembly as claimed in claim1, wherein distal ends of first and second electrodes for the dischargeof plasma gas do not project beyond the housing.
 8. An assembly asclaimed in claim 1, wherein distal ends of the first and/or secondelectrodes for the discharge of plasma gas is/are formed from graphite.9. An assembly as claimed in claim 1, further comprising cooling meansfor cooling and condensing material which has been vaporised in theprocessing zone.
 10. An assembly as claimed in claim 9, wherein thecooling means comprises a source of a cooling gas or a cooling ring. 11.An assembly as claimed in claim 1, wherein the power source forgenerating a plasma arc in the processing zone between the first andsecond electrodes comprises a DC or AC power source.
 12. A plasma arcreactor comprising a combination of a reaction chamber and a twin plasmatorch assembly according to claim
 1. 13. A reactor according to claim12, wherein the chamber has an elongate form with a plurality oforifices in a wall portion thereof; and a twin plasma torch assemblyaccording to any one of the preceding claims being mounted over eachorifice.
 14. A reactor as claimed in claim 13, wherein the chamber has atubular portion with a plurality of orifices in a wall portion thereof,a twin plasma torch assembly being mounted over each orifice.
 15. Areactor as claimed in claim 14, wherein said orifices are provided alongand/or round said tubular portion.
 16. A reactor as claimed in claim 13,wherein said orifices are provided at substantially regular intervals.17. A process for producing a powder from a feed material, which processcomprises: (A) providing a plasma arc reactor as defined in claim 12;(B) introducing a plasma gas into the processing zones between the firstand second electrodes; (C) generating a plasma arc in the processingzones between the first and second electrodes; (D) supplying feedmaterial into the plasma arcs, whereby the feed material is vaporised;(E) cooling the vaporised material to condense a powder; and (F)collecting the powder.
 18. A process as claimed in claim 17, wherein thefeed material comprises or consists of a metal or alloy.
 19. A processas claimed in claim 18, wherein the feed material is aluminium or analloy thereof.
 20. A process as claimed in claim 17, wherein the feedmaterial is in the form of a wire, fibres and/or a particulate.
 21. Aprocess as claimed in claim 17, wherein the plasma gas comprises orconsists of an inert gas.
 22. A process as claimed in claim 21, whereinthe plasma gas comprises or consists of helium and/or argon.
 23. Aprocess as claimed in claim 17, wherein at least some cooling of thevaporised material is achieved using an inert gas stream.
 24. A processas claimed in claim 17, wherein at least some cooling of the vaporisedmaterial is achieved using a reactive gas stream.
 25. A process asclaimed in claim 17, wherein the surface of the powder is oxidised usinga passivating gas stream.
 26. A process as claimed in claim 25, whereinthe passivating gas comprises an oxygen-containing gas.
 27. A process asclaimed in claim 17, wherein the powder comprises particlessubstantially all of which have a diameter of less than 200 nm,preferably less than 50 nm.